Many applications require thin, flat slices of particular materials, for example glass slices as a substrate for the production of magnetic storage disks, slices of sapphire or silicon carbide as a base for the manufacture of optoelectronic components, or semiconductor slices for the production of photovoltaic cells (“solar cells”) or as a substrate for the structuring of microelectronic or micro-electromechanical elements.
Semiconductor slices are slices of semiconductor materials such as, for example, element semiconductors (silicon, germanium), compound semiconductors (for example, aluminum or gallium) or compounds thereof (for example, Si1-xGex, 0<x<1; AlGaAs, AlGaInP etc.).
The starting material is usually in the form of a rod of monocrystalline (electronics applications) or polycrystalline (solar cells) semiconductor material, and the required slices of this material are cut off from the rod by a chip-removing process such as cut-off lapping. A particle removed from the workpiece is referred to as a chip.
In particular, for cutting off semiconductor slices, cut-off lapping and cut-off grinding are especially important. In the case of cut-off lapping, the tools for removing material are in the form of sharp-edged particles of a hard substance, for example silicon carbide, as a suspension in a viscous carrier liquid, and a tool carrier in the form of a wire, on which the carrier liquid and hard substances adhere, brings these particles into contact with the workpiece. The carrier liquids include, for example, water, polyols, mineral oils, glycols, or mixtures thereof. The suspension of hard substance is referred to as slurry. As a result of motion the wire in the longitudinal direction of the wire, exertion of a force in the transverse direction of the wire and supply of slurry, hard substances enter between the surface of the wire and that of the workpiece, are moved under pressure relative to the latter by means of a sliding or rolling motion, and remove chips from the workpiece by means of material overloading or fatigue, by brittle erosion.
It is characteristic of cut-off lapping that the slurry contains hard substances that effect removal of material, and the tool carrier does not contain any hard substances that effect removal of material, and the removal of material is based on an interaction of three bodies (firstly, the workpiece; secondly, the hard substance; thirdly, the tool carrier).
In the case of cut-off grinding, the tools that remove material are in the form of sharp-edge particles of hard substances that are fixedly bonded to a surface of a tool carrier. The tool carrier is, for example, a wire.
It is characteristic of cut-off grinding that the tool carrier contains fixedly bonded hard substances that effect removal of material, and the cooling lubricant does not contain any hard substances that effect removal of material, and the removal of material is based on an interaction of two bodies (firstly, the workpiece; secondly, hard substances fixedly bonded to the tool carrier).
The process of cut-off lapping and cut-off grinding by means of a wire is referred to by the combined term, wire sawing.
In the case of cut-off lapping and cut-off grinding, the hard substances used include, for example, silicon carbide, boron carbide, boron nitride, silicon nitride, zirconium oxide, silicon dioxide, aluminum oxide, chromium oxide, titanium nitride, tungsten carbide, titanium carbide, vanadium carbide, diamond, sapphire, and mixtures thereof. Silicon carbide is particularly important in the case of cut-off lapping, and diamond in the case of cut-off grinding.
The wire, as tool carrier, may be monofilar, or may be a stranded wire composed of a plurality of strands or fibers, and also of differing materials, and may possibly carry additional coatings of metals, alloys or plastics.
In the case of cut-off lapping and cut-off grinding, the wires are composed, for example, of hardened steel (“piano wire”), plastics, carbon fibers or metal alloys.
Both cut-off lapping and cut-off grinding may be performed with one or more wires. Examples of the latter are so-called gang saws, in which the multiplicity of individual wires are fastened in a frame (gang), which is then moved back and forth in the longitudinal direction of the wire, such that the wires work through the workpiece.
In the cutting of slices from a rod of semiconductor material, wire saws that have just one wire are of particular importance.
Rods of semiconductor material for electronics application are usually processed before the cutting operation, such that they are in the shape of a straight, circular cylinder, having a rod axis, a cylindrical circumferential surface and two cylinder end faces (bottom and top face). Usually, an identification groove (notch), which marks, for example, a particular crystal orientation, is also ground into the circumferential surface of the rod, parallel to the cylinder axis. The semiconductor slices obtained by the cutting operation are also referred to as “wafers,” and they themselves, in turn, are in the shape of a straight, circular cylinder, the height of the cylinder being some tenths of a millimeter up to, for instance, one millimeter, and the base of the cylinder having a diameter of 75 to 450 mm. Rods or wafers having a diameter of up to 150 mm are classed as small, those up to 200 mm as medium-sized, those up to 300 mm as large, and those up to 450 mm as very large.
Dividing up into slices, by means of cut-off lapping or cut-off grinding, is effected along a multiplicity of kerfs, which are as flat as possible and as parallel to each other as possible, and which are substantially perpendicular to the rod axis, i.e. with a deviation of up to 2° relative to the perpendicular to the rod axis.
Cut-off lapping and cut-off grinding are equally suitable for dividing up small and medium semiconductor rods. For dividing up large and very large semiconductor rods, cut-off lapping is particularly important, since, even in the case of long lengths of wire in contact with the rod, it produces wafers whose front and back have a high degree of flatness and parallelism in relation to each other. In addition, the cutting faces that then form the front and back of the cut-off slices have only a small depth of crystalline damage caused by the cutting operation.
Owing to the shallowness of the damage, wafers obtained by cut-off lapping are less susceptible to breakage, and during the subsequent processing only a small amount of material then has to be removed in order ultimately to obtain a wafer having the required high degree of plane parallelism of the front and back, and lack of defects. Cut-off lapping therefore enables particularly high-quality wafers to be produced in a particularly cost-effective manner.
The length along which a wire portion extends through the workpiece at any moment in the cutting operation, and on which the wire portion thus acts to remove material, is referred to as the engagement length of the wire portion.
In the following “multiwire cut-off lapping” ([slurry] multiwire slicing, MWS, S-MWS) is described in greater detail using the references from FIG. 1.
In cut-off lapping, the wire 1 is wound helically around at least two cylindrical wire guide rollers 3 and 4, which have cylinder axes 5 and 6 that are parallel to each other, such that, on at least one side of this arrangement, a multiplicity of portions 11 of the wire come to lie (25) parallel to each other in one plane, perpendicularly in relation to the cylinder axes, and when the wire guide rollers are rotated 7 and 8 in the same direction, about their respective axes, the wire portions 11 move with a uniform velocity and parallel to each other in the longitudinal direction of the wire. The term wire grid—or sawing grid—is used to refer to this at least one side of wire portions running parallel to each other. The position of the wire grid in this case is selected such that it faces toward the rod to be cut.
The wire guide rollers in this case are normally each provided with a multiplicity of closed grooves 2, which are parallel to each other and perpendicular in relation to the cylinder axis, and which are in flush alignment in respect of adjacent guide rollers, and in which the individual turns of the wire are guided.
Removal of material is effected by moving the wire portions in the longitudinal direction of the wire, supplying the suspension, comprising the hard substances, to the wire grid, and advancing the workpiece on to and through the wire grid.
In the run-in to and run-out from the wire grid, the longitudinal wire tension in the running direction of the wire is controlled by means of deflection rollers, which are attached to levers, an alteration of the angle of the levers in relation to the longitudinal direction of the wire causing the running length of the wire to be changed, and thus enabling the wire to be tensioned to a greater or lesser degree.
The torque exerted upon the lever by the wire provides a measure of the actual wire tension, such that, by means of torque measurement and angle adjustment, there is a closed loop for feedback control of the longitudinal wire tension. Levers having a deflection roller are referred to as “dancers” because of their rapid back and forth motion as control deviations occur.
During the cut, the wire is subject to wear resulting from abrasion. The wire cross section decreases approximately in proportion to the product from the cumulative length of the wire contact with the workpiece 15 and the workpiece volume machined per kerf. Consequently, the width of the kerfs 13 decreases from the first end face 12 of the rod, at which the wire first enters a first kerf, to the opposite, second end face 24 of the rod, at which the wire ultimately emerges from the last kerf.
The first end face is also referred to as the wire infeed side 12 of the rod, and the second end face 24 as the wire run-out side.
The decrease in thickness of the wire is generally compensated by a gradual decrease in the distance between adjacent grooves of the wire guide rollers 3 and 4 from the wire infeed side 12 to the wire run-out side 24 of the rod, such that, averaged over the full length of the rod, the wafers cut off from the rod are of a constant thickness.
During the cutting operation, the wire may be wound continuously in one direction from a pay-off coil (delivery coil), via the wire guide rollers and the grid, to a take-up coil (receiver coil), such that the wire portions move in one direction throughout the entire cutting operation. This is referred to as unidirectional sawing.
The wire may also be guided through the workpiece with a change of direction. Such a bidirectional cut may be effected over the entire run of the wire supply from the pay-off coil, via the grid and workpiece, to the take-up coil, and then in full from the original take-up coil, which thus becomes the pay-off coil, back to the original pay-off coil, which now becomes the take-up coil. Owing to the wear on the wire, however, the workpieces would become thicker on the “return pass” than on the “outgoing pass,” which is undesirable.
For the production of large and very large wafers for particularly demanding applications, particular importance attaches to cut-off lapping with multiple and continuous reversal of the direction of the wire run, according to the so-called pilgrim step method (“pilgrim step motion,” “wire reciprocation”).
A pilgrim step in this case refers to a pair of successive reversals of the wire direction. A pilgrim step comprises a first motion of the wire in a first longitudinal direction of the wire, by a first length, and then a second motion of the wire in a second direction, exactly opposite to the first direction, by a second length, the second length being selected so as to be less than the first length.
For each pilgrim step, therefore, a wire length corresponding to the sum of the two lengths goes through the workpiece, while the wire portion coming into cutting engagement with the workpiece in this case advances from the pay-off coil to the take-up coil only by an amount corresponding to the difference of the two lengths. In the case of the pilgrim step method, therefore, the wire is used multiple times, by the factor resulting from the ratio of the sum to the difference of the two lengths. For simplicity, the difference of the two lengths is to be referred to as the “net motion” of the wire over a complete pilgrim step, having a net wire infeed 9 and a net wire run-out 10 (FIG. 1).
By analogy with the terms used for a rod cut by a unidirectional wire motion, in the case of a rod cut with a pilgrim step method the first end face of the rod, at which the wire first enters the first kerf, in the direction of its net motion, is referred to as the wire infeed side of the rod, and the opposite, second end face, at which the wire ultimately emerges from the final kerf, in the direction of its net motion, is referred to as the wire run-out side of the rod.
Since the length of the forward and return motion, and therefore the effective length and wear of the wire, are freely selectable, the pilgrim step method is highly suitable for cutting workpieces into a smaller number of slices, of comparatively shorter workpieces, which could not be cut with an economic use of wire by means of only a single wire pass (unidirectional cut). The pilgrim step method is particularly suitable for cutting workpieces that have wire engagement lengths that vary over the course of the cut, i.e. for example cylindrical semiconductor rods.
For the purpose of cutting wafers from a semiconductor rod, the rod is first mounted, depending on its total length, with a part of its circumferential surface on a holding, mounting or sawing strip, for example a strip made of hard carbon, glass, plastic or a composite material. This sawing strip is shaped, on the side thereof that faces away from the rod, or is connected to a further adapter, such that the sawing strip or adapter can be clamped in a corresponding receiving device, which is fixed to the feed table that, during the cutting operation, feeds the bar perpendicularly on to the grid and through the latter. The bond between the rod and the sawing strip is produced by adhesion, and that between the sawing strip and the adapter is produced by adhesion, positive fit by force or positive fit by shape (for example, clamping or screwed connection). The rod axis is aligned substantially perpendicularly in relation to the feed direction, and perpendicularly in relation to the direction of the wires in the grid, and thus substantially in the plane spanned by the wire portions of the grid.
The feed table is usually disposed above the wire grid, and feeds the clamped-in rod perpendicularly on to the plane spanned by the wire portions of the grid.
The following continues with reference to FIG. 1. The side of the cylinder surface of the rod, into which the wire portions of the grid enter, along their longitudinal direction, into the kerfs, is referred to as the (momentary) wire entry side 17, and the side on which they re-emerge from the kerfs, along their longitudinal direction, is referred to as the (momentary) wire exit side 18. Disposed above the wire grid 25 on the wire entry side, parallel to the rod axis 14, is a nozzle strip 19, having slurry nozzles 21, which extends over the entire length of the wire grid and uniformly applies slurry 22 to the wire portions 11 before they enter the rod.
In the case of a unidirectional cut, with only one wire entry side, one nozzle strip is provided for this purpose, and in the case of a bidirectional cut, with wire entry and exit sides alternating in time, two nozzle strips 22 and 23 are provided, one on each side of the rod. The two nozzle strips 19 and 20 may be operated alternately, such that the strip on the instantaneous wire entry side is active in each case, or, for simplicity, both may also be operated continuously.
As a result of the rod being fed on to the grid, the entire length of the rod is brought into contact with the grid, along a line on the cylinder surface of the rod that is parallel to the rod axis. This instant of first contact of the wire portions with the rod is referred to as the cut-in operation or, in short, cut-in. Upon further feeding, motion of the wire portions of the grid in the longitudinal direction of the wire, and supply of slurry, the wire portions work slowly through the rod, removing material.
The cut terminates as soon as all wire portions of the grid have swept over the entire cross section of the rod and have fully reached the sawing strip. The instant of final contact of the wire portions with the rod is referred to as the out-cut operation, or, in short, out-cut. In the example shown in FIG. 1, the cut is being effected to the side having the identification groove 26, from the opposite side of the rod. The mounting strip holding the rod is bonded to the latter on the side with the identification groove 26 (not shown).
The feeding of the rod is ended, and the rod is again slowly withdrawn from the grid. As the rod is being withdrawn, the wire continues to travel in its longitudinal direction, at least slowly, in order to prevent the wire portions from becoming caught on any irregularities of the previously produced cut faces.
After the rod has been withdrawn from the wire grid, the composite consisting of sawn-up rod, sawing and mounting strip is removed from the clamping device on the feed table. Thus, after completion of the cut, a multiplicity of wafers hang, like teeth on a comb, on the partially cut-in sawing strip, with a portion of their circumferential surface still joined to the sawing strip. The wafers are separated by dissolving the adhesive bond. The bond can be dissolved, for example, if an adhesive has been used that can be dissolved by water or heat, the composite consisting of wafers, sawing and mounting strip being immersed in a hot water-bath for the purpose of so-called degluing.
Slurry cut-off lapping, and a suitable apparatus therefor, for cutting off semiconductor slices are described, for example, in EP 0 798 091 A2.
The width of the kerf produced, and consequently the thickness of the wafers obtained by the cutting operation, depends on the thickness of the wire, the thickness of the slurry film surrounding the wire in the kerf, and the spacing of the grooves in the wire guide rollers that guide the wire. Since the thickness of the wire changes continuously because of wear, and the thickness of the slurry film changes continuously as a result of being wiped off or becoming spent during the cutting operation, cut-off lapping is subject to certain limitations in respect of the degree to which the desired shape of the wafers obtained thereby can accurately be achieved. These limitations are described in the following.
As a wire portion enters the workpiece, most of the adhering slurry is stripped off. Of the slurry that does enter the kerf, most continues to be stripped off or become spent as the wire penetrates further into the rod. Wear causes the particles to become spent primarily because the hard materials become broken or fragmented, or because rounding or chipping off causes them to lose their sharp-edged cutting surfaces that remove material. The decrease in thickness of the slurry film surrounding the sawing wire, as viewed in the direction of entry of the wire, is also referred to as the “slurry funnel”. Owing to the slurry funnel, each kerf is wide at the periphery of the workpiece, on the side on which the wire enters, and tapers in a wedge or funnel shape in the running direction of the wire, along the wire engagement length, to the opposite, wire exit side.
In the case of a unidirectional cut and large or very large semiconductor rods, in particular in the region of the longest wire engagement length—thus, in the case of a circular cylindrical rod, when the latter has been cut through by exactly half—virtually no more slurry reaches the wire exit side of the rod. Slices are produced having a thickness that increases in a wedge shape from the wire entry side to the wire exit side, with a high degree of roughness on the wire exit side (“sawing scores,” “saw marks”). Semiconductor slices that have a tapering thickness and a high degree of roughness are not suitable for demanding applications. Unidirectional cut-off lapping therefore cannot be used for producing high-quality large or very large wafers.
In the case of the pilgrim step method, the direction of the longitudinal motion of the wire is reversed continuously. As a result, “slurry funnels” are produced alternately on the left and right in each kerf. If the infeed of the rod is effected so slowly, or the change in direction of the longitudinal motion of the wire is effected so rapidly, that the slurry funnels formed consecutively in alternating fashion overlap somewhat every second change of direction (=one complete pilgrim step)—as viewed in the direction of advance of the rod—the kerf is effectively supplied with slurry on both sides, and the slurry then only has to be transported as far as the center of the rod in each case. The overlapping also reduces the widening of the kerfs on the wire entry side that is caused by the slurry funnel in each case, and thereby reduces tapering of the wafers. The wafers obtained are then no longer wedge-shaped, but still have a slight saddle shape, with thicknesses that decrease slightly from the center to their peripheries, in both running directions of the wire. The minimum thickness occurs in the peripheral region of greatest wire engagement length, i.e. when a circular cylindrical has been cut through exactly by half. The zones of minimum thickness in the peripheral region of greatest wire engagement length are denoted by 27 and 29 in FIG. 2. Owing to the net motion of the wire in the case of cut-off lapping according to the pilgrim step method, and the decrease in thickness that the wire undergoes in this process, the taper 27 of the wafer on the wire entry side is somewhat thicker than the taper 29 on the wire exit side; however, both are considerably less than the wedge shape of a comparative cut with a unidirectional wire motion.
This saddle shape can be rendered less pronounced by methods known in the prior art, for example by adjusting the first and second wire motion lengths of the pilgrim steps according to the actual depth of cut (depth of the kerf in the direction of the ingot feed), or by increasing the wire application in the region in which the saddle shape is most pronounced. In particular, for example, the rod feed rate can be reduced to such an extent that the slurry funnels of two wire reversals overlap almost completely. There are economic efficiency limits to the measures for reducing the saddle shape, since they result in very long overall cutting times and high wire-length consumption rates.
A further limitation of the flatness, of the front and back of slices obtained from rods having wire engagement lengths that vary over the cut, that can be achieved by cut-off lapping consists in the form of a so-called “cut-in wedge” 28 (FIG. 2). This cut-in wedge comprises a slope, of the front and back in the transverse direction of the wire and toward the central plane of the slice, which extends over the entire slice width in the longitudinal direction of the wire.
In the case of cylindrical rods, the cut-in wedge is produced because, for example, at the instant at which the wire grid first makes contact with the rod, the engagement length of the wire portions and also, consequently, the wear on the wire are zero.
Since they initially have not been subjected to any wear, the wire portions that cut in first have a greater diameter than the wire portions that come into engagement later, which have already passed through kerfs of finite length and have therefore been subjected to wear. As the cut-in depth increases, therefore, the result is a wedge-shaped increased thickness 28 of the wafers. In addition, at the instant at which the wire portions first make contact with a cylindrical rod, the wire portions are tangential to the circumferential surface of the rod. At that point, less slurry is wiped off than as the cut progresses further, when the engagement lengths are finite and the contact of the wire portions with the circumferential surface of the rod becomes increasingly steeper—even precisely perpendicular in the region of greatest engagement length (half cut)—and a regular surge of stripped-off slurries builds up there, at the point where each wire portion enters the rod.
WO 2013/051183 A1 describes wire cut-off lapping by the pilgrim step method, in which conical wire guide rollers are used, the distance of the base of the grooves from the axis of the wire guide roller decreasing from the wire infeed side to the wire run-out side. As a result, when the rod with the rod axis is fed perpendicularly on to the wire guide roller axes, the wire portions of the wire grid that are on the wire infeed side are the first to come into engagement with the rod. Owing to the net motion of the wire, these wire portions advance from the wire infeed to the wire run-out side of the wire guide roller and have become thinner by abrasion, as a result of the rod being fed further on to the grid, until ultimately the part of the rod on the wire run-out side also comes into material-removing engagement with the wire grid, such that the formation of the cut-in wedge is counteracted.
Owing to the conical shape of the wire guide rollers, the lengths for respectively one winding around the wire guide roller decrease from groove to groove, from the wire infeed side to the wire run-out side. All grooves of each wire guide roller rotate with the same angular velocity, but not with a constant circumferential velocity, owing to the decrease in groove diameter from the wire infeed side to the wire run-out side. Since the wire is moved in the grid by the circumferential velocity of the grooves, however, the axial wire tension in the grid is therefore reduced from the wire infeed side to the wire run-out side.
It is known to persons skilled in the art who are familiar with wire cutting methods that the flatness of the kerf that is produced decreases as the longitudinal wire tension decreases. The method described therefore produces slices whose flatness worsens toward the wire run-out side.
This fundamental method deficiency also cannot be eliminated by, for example, the obvious means of using four wire guide rollers, selected so as to complement each other exactly, of which the upper two, which span the grid of the wire portions coming into contact with the rod, are provided with a groove depth that increases from the wire infeed side to the wire run-out side, and the lower two are provided with a groove depth that decreases from the wire infeed side to the wire run-out side. Although the lengths of the wire windings would then be constant over the entire length of the wire guide rollers, there would nevertheless be a pronounced forced slippage of the wires in the grooves, between the upper and the lower wire guide rollers, owing to the differing circumferential velocities, resulting from the differing diameters, at the same angular velocities. As a result of this, the grooves of the wire guide rollers would be subject to a large amount of wear and would consequently undergo differing cut-in by abrasion by the wire, with the result that the anticipated advantages of the method would already be eliminated in a very short time. WO 2013/051183 A1 does not discuss this putative solution to the problem of wear.
Moreover, an uneven wire tension in the grid has the effect that the wire longitudinal tensions of the wire portions must be selected so as to be very low, on average, over the grid, since the most highly tensioned wire portions, on the wire infeed side, may have at most the wire longitudinal tension at which the wire is still certain not to rupture (tensile strength), and therefore all other wire portions have a lesser tension that these wire portions. Consequently, the majority of the wire portions undergo wire flexure that increases toward the wire run-out side, with increasingly poorer wire guidance and worsening flatness of the wafers obtained.
Although the unevenness of the wire tension over the wire grid will be compensated somewhat by wire slippage, the method according to WO 2013/051183 A1 is therefore nevertheless unsuitable for execution according to the pilgrim step principle, because, in the second half of each pilgrim step, wire portions of short winding lengths on the wire run-out side would return to long winding lengths on the wire infeed side, causing them to become overtensioned and rupture there. This could be counteracted only by reducing yet further the wire longitudinal tension with which the wire is supplied to the grid in a controlled manner (setpoint longitudinal wire tension), in order to ensure a sufficiently reliable wire tension reserve (“headroom”), up to the wire rupture tension, in order to prevent wire ruptures during the cutting operation. This, however, would involve a yet more impaired flatness of the wafers obtained, owing to the imprecise and unreliable wire guidance in the kerfs.
The method specified in WO 2013/051183 A1 is therefore unsuitable for producing highly planar semiconductor slices for demanding applications.
U.S. Pat. No. 8,146,581 B2 discloses a method for wire cut-off lapping in which the axis of the rod is tilted slightly, in the feed direction, toward the axis of the wire guide rollers. In this case, the wire feed rollers have a cylindrical shape, their axes are disposed so as to be parallel to each other, and all grooves for guiding the sawing wire are incised to a uniform depth. When the rod is fed in therefore, an end-face rod end is cut into first and the opposite rod end is cut into last, owing to the tilting of the rod axis in relation to the axes of the wire guide rollers.
Since the circumferential velocities of all wire portions are equal, this method does avoid the problems resulting from wire tensions varying over the wire grid, as well as excessive groove wear caused by wire slippage; however, the axial tilt results in an additional force in the transverse direction of the wire and parallel to the rod axis (slope downforce). The slope downforce acts only in the instant of the respective cut-in of each wire portion, and vanishes as soon as the cut-in has been made. The cut face therefore runs with wires that, in the region of the cut-in, are deflected out of the cut plane produced by a cut without transverse force, and is therefore curved in that region. Since the slope downforce upon adjacent wire portions is approximately the same, the front and back of the sliced obtained are substantially parallel to each other, and have a uniform curvature for the front and back in the cut-in region. The slices obtained are thus of a substantially constant thickness, but have a curved or warped shape (“cut-in wave”).
U.S. Pat. No. 8,146,581 B2 utilizes this cut-in wave, deliberately produced by tilting, to compensate a random cut-in wave that might be present on a wafer cut previously without a slope downforce. Obviously, the method may also be used to counteract a cut-in wedge, since, owing to the tilt, the individual wire portions effect cut-in in time succession along the rod axis, and are thus subjected to differing wear. This possible application is not identified by U.S. Pat. No. 8,146,581 B2.
Since, in the method according to U.S. Pat. No. 8,146,581 B2, owing to the rod tilt, cutting off is effected along planes that are tilted against the rod axis, the slices that are obtained have an unwanted misorientation. This misorientation could be compensated by correspondingly tilting the crystal axis in the opposite direction, against the rotation axis, during the preceding circular grinding of the rod. For this, however, it would be necessary to know in advance during circular grinding, for each rod, what tilt, and by what angle, would subsequently be necessary for a wire saw in order that the instantaneous inclination of this wire saw, for forming an cut-in wave, could be compensated precisely without the slices obtained having a misorientation. Clearly, for reasons of causality, this is not possible.
Slices cut off according to U.S. Pat. No. 8,146,581 B2 therefore always have an unwanted misorientation, such that the method is likewise unsuitable for producing slices for demanding applications.